Wellbores are drilled by a drill bit coupled to the end portion of a drill pipe. The drill bit drills the wellbore to a “pilot hole” diameter. During or after the drilling of the wellbore to the pilot hole diameter, an underreamer may be used to enlarge the diameter of the wellbore from the original “pilot hole” diameter. The underreamer is run into the wellbore on the same drill pipe, behind the drill bill. The underreamer actuates between an inactive state and an active state. In the inactive state, cutter blocks on the underreamer are folded or retracted inwardly into the body of the underreamer such that the cutter blocks are positioned radially-inward from the surrounding casing or wellbore wall. Once the underreamer reaches the desired depth in the wellbore, the underreamer is actuated to an active state. In the active state, the cutter blocks move radially-outward and into contact with the wellbore wall. The cutter blocks are then used to increase the diameter of the wellbore.
Underreamers are generally spaced axially apart from the drill bit on the drill pipe. For example, the underreamer is typically positioned “above” the drill bit by about 30 m to about 60 m. As such, the underreamer is not able to increase the diameter of this lower portion (30 m-60 m) of the wellbore because the drill bit contacts the subterranean formation proximate the base of the wellbore, thereby preventing further downward movement of the underreamer. This portion of the wellbore that remains at the pilot hole diameter is called the “rat hole.” What is needed, therefore, is an improved system and method for increasing the diameter of at least a portion of the rat hole.